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Difference between Petrol Engines and Diesel Engines

Petrol Engines vs Diesel Engines

One of the confusions prevailing among the car buyers is the difference between the petrol (gasoline) engine and the diesel engine. Perhaps the common knowledge or perception is that the diesel motors are noisier and their vibration is intolerable, whereas petrol motors run smoothly. Well this is not far from the truth though. But, with the modern engine technology, diesel engines are more competitive than before, so we have cars running efficiently, and we also have an increasing popularity and demand.

Petrol and diesel engines are the most popular internal combustion engines used predominantly in modern vehicles, with the diesel also heavily used in construction industry to manage heavy loads. We will unfold the differences between these engines. Let’s start with how they work.

How do gasoline and diesel engines work?

These engines seemingly operate the same with the difference seen in the way they ignite the fuel. They use the same 4 stroke – a series of steps that include the intake, compression, combustion/power and exhaust. The main aim is to convert the chemical energy in respective fuels into mechanical energy for the car wheel to be activated.

Petrol Engine

In a bit of history, petrol engine was invented in 1876 by Nikolaus August Otto. This explains why petrol engine is said to run on Otto cycle which is a 4-stroke combustion cycle. Other engines came in after this invention aimed at improving its efficiency. Further assessment of the petrol engine revealed that about 10% of fuel was being used, whereas the rest was just lost in the production of a needless heat. But since then modern technologies have played a pivotal in revolutionizing the way gasoline works.

Petrol engines use the electrical spark to ignite the fuel and air mixture in the combustion chamber. There is a piston in linear motion; that is going down and up when prompted to. When it goes down, it sucks in air and the fuel is also added at the same time to create a smooth mixture in the carburetor before it goes to the cylinder. Petrol is highly volatile and evaporates than diesel. So it is easy to mix with air.

The piston will then go up to compress the mixture of air and petrol fuel in the cylinder. That compression will make the mixture warmer, but not warm enough for self-ignition as is the case with the diesel. If this were to be a case, the mixture may react spontaneously and cause engine knock, which will then damage the engine components.

To ignite the mixture during the movement of the piston, the spark plug is used. The mixture will burn very fast while turning the liquid mixture into a gas. The power stroke results wherein the increase in volume lets the piston forcefully down. As it goes up, the valves open will make way for the exhaust. The process continues like that for gasoline engines, including those ones with the cylinder direct petrol injection technology. The gasoline engine 4-stroke combustion cycle is outlined below:

  • Intake stroke – the fuel is mixed with the air in the carburetor

  • Compression stroke – the mixture of fuel and air is compressed when the piston goes up in the cylinder

  • Ignition stroke – the spark plug is used to ignite the mixture of fuel and air

  • Exhaust stroke – the piston will push the exhaust out through the exhaust valve

Diesel Engine

Two years after the invention of petrol engine, Rudolf Diesel, while attending an engineering college in Germany in 1878, learned about the low efficiency of gasoline engine, and then got inspired to invent a powerful competitor – the diesel engine, to provide a higher efficiency with regard to combustion power. The diesel engine was patented in 1892.

Unlike the petrol engine, the diesel engine does not rely on the spark plug to ignite its fuel. What it does is to rely on high pressure compression to make the explosion. There is still a movement of the pistol in the engine wherein it sucks in the air to be compressed. When it goes up, the piston, it compresses the air using a higher compression ratio ranging from 14:1 to 25:1 compared to the 8:1 to 12:1 compression ratio of the petrol engine.

If the engine has a turbocharger, it will suck in more air forcefully into the cylinder and exert more pressure. The heat in the cylinder can reach very high temperatures. The fuel injector will inject diesel fuel just in time, and it will start to burn as a result of the high-air temperature and pressure in the cylinder. This combustion will then produce an enormous amount of gas which will keep the wheels rolling. Diesel engine is also using the 4-stroke combustion cycle as outlined below:

  • Intake stroke – the intake valve lets in air when the piston goes down

  • Compression stroke – because the piston continues with the up and down motion, it will lead to the compression stroke when it goes up

  • Combustion stroke – the fuel will be injected to be ignited by the high temperate and pressure, thus also forcing the piston down again

  • Exhaust stroke – as the piston goes up again, it lets out the exhaust through the exhaust valve

Key differences between these two engines

To better understand these differences clearly, we have to take into account certain aspects such as the combustion, fuel efficiency, eco-friendliness, engine powers/speeds, and costs. The petrol fuel, not the engine, is inevitably expensive but avid car lovers continue buying petrol engines. Why is that? That has to do with the advantages of the petrol engine over the diesel. Likewise, diesel is perceived to be slow and noisy, yet some people, particularly the construction and farming industries keep relying on it. That also takes into account the advantages that outweigh the disadvantages.

Combustion

The first difference is seen in the way combustion is being run between these two engines. As already highlighted, combustion is ignited by the spark plug in a gasoline engine, whereas the high compressed air ignites the fuel in a diesel engine. Diesel tends to have a higher compression ratio and that gives it more torque and ability to haul heavy loads.

Because of the higher compression ratio, the diesel engine needs more rugged engine components. So it has heavy components. This explains why it is not favorably used in airplanes and racing cars because it may compromise their intended speed. The diesel engine is, nonetheless, heavily used in buses, trains, boats and trucks for the need of high power in these vehicles.

On the other hand, petrol engine has a low compression ratio. This can be attributed to volatility of the petrol fuel when mixed with air because it can cause engine knock, which can eventually damage the engine. The low compression ratio requires no heavy engine components. As a result of this, the petrol engine is often in light cars for its high horse power and speed.

Fuel efficiency

With regard to fuel efficiency, cars running on diesel engine are more fuel efficient. Petrol evaporates more and releases energy fast. For this reason, you will have many trips to the filling station, and on top of that the petrol fuel is expensive.

On an overall, the diesel engines have higher mileage than the petrol engines, especially for long distances. Moreover, its fuel is cheaper, so will save more even though the diesel cars are relatively expensive than petrol cars.

Maintenance

Petrol engines are not maintained frequently but do not last. Petrol reduces the lubrication, so the engine components wear faster. The diesel engine lifespan is almost twice that of the gasoline engine. Diesel engines also do not have frequent maintenance need but you must frequently replace the oils and filters; otherwise the engine could be damaged. The diesel engine maintenance is more expensive than petrol engine.

Performance and Speed

Diesel engine performs better where power is needed because of its more torque. So for loading heavy machinery on your vehicle, you need a diesel engine. But it lacks when coming to speed. Petrol engines outsmart the diesel engines with speed because of their increased horsepower. They are lightweight, so the petrol cars will run faster. But petrol engines are not ideal for loading heaving machinery on your vehicle.

Diesel engines can have a starting problem when the engine is too cold to ignite the fuel. In such cases, the glow plug is used as the electrically heated wire to heat the combustion chamber so that self-ignition can be initiated under higher temperatures. But modern technology has brought the computer controls wherein the engine is controlled by the ECM which communicates with sensors that aim to solve the issue of ambient temperature to enhance the self-ignition of the engine. But small engines still rely mainly on glow plug.

Eco-friendliness

Unlike gasoline engines, diesel engines emit less carbon monoxide/dioxide to the environment even though the burning of diesel is more visible than that of the gasoline. But the con side of diesel is the emission of large amounts of nitrogen compounds which can affect your health.

Price matters

Diesel fuel (C14H30), evaporates slowly because it is non-volatile. It is also refined easily than a petrol fuel (C9H20). This explains why diesel is much cheaper than petrol. However, the same cannot be said with the diesel engine and the petrol engine. The diesel engine is expensive to buy and to maintain compared to the petrol engine. Even when purchasing the diesel car, you will break your bank. Nonetheless, the difference may be recouped from a filling station because the diesel fuel doesn’t get depleted faster and it is cheaper.

Can I put diesel fuel in petrol engine and vice versa?

This may be a common question asked by many people out there concerned with the difference between these engines. Diesel is less volatile, so it will not react with air as much as petrol does. The spark applied to poor mixture will not even succeed in forming combustion.

As for the petrol in the diesel engine, it may result with detonations because of its high volatility under the high compression ratio of the diesel engine. The engine might be damaged severely. Furthermore, petrol has no lubricant properties, so the components of the engine might wear out, thus leading to an expensive maintenance.

Tabular comparison between diesel engine and petrol engine

Diesel Engine

Petrol Engine

Works on Diesel cycle

Works on Otto cycle

Air is compressed and the fuel injector used to spray the fuel for self-ignition combustion

Fuel and air mixture in the carburetor and ignited by the spark plug

High compression ratio and high torque

Low compression ratio and low torque

More fuel efficient

Less fuel efficient

Non-volatile and evaporates slower and has a high flash point

Volatile, evaporates faster and has a low flash point

Heavyweight, so used in heavy machinery and heavy vehicles such as buses, boats and trucks

Lightweight, so used in light cars such as sports cars, motorbikes, and airplanes

Expensive to maintain but is durable

Less expensive to maintain but does not last

Can haul heavy loads because of its high power density

Not fit to haul heavy loads because of its less power density

Diesel fuel cheaper

Petrol fuel expensive

Diesel cars expensive

Petrol cars affordable

Verdict

The choice of any internal combustion engine between petrol and diesel is purely a personal preference. It depends on the use of the car. If you want to indulge in racing sports, go for petrol engines. Very light and high speed it is. The car is inexpensive compared to the diesel car. But the diesel car is more powerful especially in heavy loads. The diesel fuel and diesel engine are indispensable in the economy because the construction and farming industry could suffer tremendously in their absence.

Wrap up!

The diesel engine and gasoline engines are often referred to as CI- Combustion Ignition and SI – Spark ignition engines. These are the popular internal combustion engines. We saw how different they are, particularly in fuel ignition, as well as in other aspects such as fuel economy, environmental effects, speed and power, and the costs of maintenance.

Regardless of the difference, they use the same 4-stroke combustion cycle with petrol engine’s one denoted Otto cycle and diesel engine, the diesel cycle. The difference is, as aforementioned, the fuel ignition because petrol engine uses spark plug, whereas the diesel engine solely uses high compression to self-ignite.


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